A Genetic View of Human Origins
Svante Pääbo
Abstract
The two big questions for people interested in the history of our species concern how we humans came to be and how we are different from our closest living relatives, the African apes. The advent of new molecular genetic tools means that this topic can now be addressed more powerfully than ever before. The complete genomes of humans and chimpanzees are in hand, with other primate species to follow. Even more exciting are the proliferation of studies of gene expression, which looks at how the genetic code is employed differently in various tissues and species. It is now even possible to examine evolution in action, through the study of ancient DNA.
Long before modern humans left Africa, western Asia and Europe were peopled by a more archaic form of humans, the Neanderthals. We have developed techniques for the retrieval of DNA from archaeological and paleontological remains and used these to study Neandertals. Our results suggest that Neandertals diverged from the ancestors of modern humans about half a million years ago. Together with other findings, this supports a scenario in which modern humans originated from small African populations that colonized the rest of the world rather recently without mixing much with resident archaic humans. We are currently developing techniques that will make it possible to sequence the entire Neandertal genome. This will allow us to identify almost all genetic differences that set modern humans apart from the Neandertals.
The comparative analysis of genomes and gene function in humans and the great apes allows us to directly see mutations in the genetic code, and see which ones have led to measurable differences in gene activity. Our results so far show that the male germ line has been the target of much positive selection in both human and chimpanzee ancestors while the there are indications of positive selection also in the brain but only in humans and not in chimpanzees. Among genes that may have influenced recent human evolution, FOXP2 stand out since when mutated it causes severe language and speech problems in humans. We have shown that FOXP2 was the target of strong positive selection in humans during the last 250,000 years, i.e. substantially later than the divergence between anatomically modern humans and Neandertals. This is compatible with the idea that articulate language was a trait that distinguished modern humans and Neanderthals. In order to understand the phenotypic effects of the changes that were positively selected in FOXP2 we are currently establishing animal models to study its effects.
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